Process for the preparation of hexafluoropropene



United States Patent 3,397,248 PROCESS FOR THE PREPARATION OF HEXAFLUOROPROPENE Donald George Hummel, Wilmington, Del., and Frederick Wurl Swamer, Boothwyn, Pa., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed May 15, 1964, Ser. No. 367,894 2 Claims. (Cl. 260-6535) ABSTRACT OF THE DISCLOSURE Production of 'hexafluoropropene by the pyrolysis of 2- chloro-l,1,l,3,3,3 hexaflnoropropane.

Background of the invention This invention is directed to a novel process for the preparation of hexafluoropropene. More particularly, the present invention is directed to the production of hexafluoropropene by the pyrolysis of 2-chloro-l,l,l,3,3,3- hex-afluoropropane. By pyrolysis is meant the conversion of the halopropane into hexafluoropropene through the agency of heat alone.

Hexafiuoropropene has heretofore been prepared by dehalogenation, decarboxylation, and pyrolysis reactions. In one such reaction ootafluoropropane is heated at temperatures between 1000 C. and 1150 C. in contact with a heated platinum filament to yield mostly hexafluoroethane with small amounts of hexafluoropropene. In another procedure small amounts of hexafluoropropene are produced by the pyrolysis of tetrafluoroethylene at 600 C. to 700 C. Hexafluoropropene has also been produced as a byproduct in the synthesis of tetrafiuoroethylene from chlorodifluoromethane. A more economical process to provide hexafluoropropene in higher yields and larger quantities has been needed.

It is an object of this invention to produce hexafluoropropene by a simple, economical and efiicient method. A further object is to provide hexafiuoropropene in good yield from a readily attainable starting material. These and other objects will become readily apparent from the following description and examples. These objects are accomplished by the pyrolysis of 2-chloro-1,1,l,3,3,3-hexafluoropropane at elevated temperatures to produce hexafiuoropropene.

Summary of the invention More specifically the present invention is directed to a process for producing hexafiuoropropene which comprises pyrolyzing 2-chloro-l,l,l,3,3,3-hex-afluoropropane at a temperature in the range of from about 650 C. to about 850 C., for a time in the range of from about 3 seconds to about 120 seconds and isolating the hexafluoropropene produced.

Description of the invention The pyrolysis is effected in an elongated tube constructed of or lined with -a metal that is inert to the acidic reaction products. Suitable metals for such a reactor are silver, a high temperature corrosion resistant alloy of nickel chromium, and iron or alloy of nickel, molybdenum and iron. A tube of the nickel, chromium, and iron alloy lined with platinum provides a reactor having a longer life than the alloy alone. The dimensions of the tube will depend upon the manner and scale of operation commensurate with maintaining the required temperature across! the tube with a velocity of feed gas through the tube that permits the required contact time with the heated zone. The tube, for example, may be narrow and long, relatively wide and short, or have intermediate dimensions.

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Any convenient source of heat may be used to maintain a temperature in the range of from between about 650 C. to about 850 C. :along a length of the tube. The preferred temperature range is from about 725 C. to about 775 C. maintained with electric heating.

The 2-chloro-1,1,l,3,3,3-hexafluoropropane starting material is produced by reacting carbon tetrachloride with trichloroethylene or reacting chloroform with tetrachloroethylene to form hexachloropropene with the liberation of hydrogen chloride. The hexachloropropene is then reacted in contact with an activated anhydrous chromium oxide catalyst with hydrogen fluoride to provide 2-chloro-1,1,1, 3,3,3-hexafluoropropane, in a manner described for forming chlorofiuoroalk anes in United States patent application Ser. No. 833,962, filed on Aug. 17, 1959 and assigned to E. I. du Pont de Nemours and Company.

The 2-chloro-1,l,1,3,3,3-hexafluoropropane (B.P. of about 15 C.) may be passed directly as an undiluted gas into the reactor or it may be mixed with an inert gas such as nitrogen or helium. Use of the reactant for pyrolysis in unmixed form is preferred. Usually the gaseous chlorohexafluoropropane is fed to the reactor at about atmospheric pressure and at room temperature. Higher pressures and subatmospheric pressures may be used and the gas may be preheated up to 600 C. The feed rate of the 2-chloro-1,1,l,3,3,3-hexafluoropropane is regulated in accordance with the dimensions of the reactor to give a contact time with the heated zone in the range of from about 3 seconds to about seconds. A cont-act time in the range of from about 20 seconds to about 30 seconds is preferred. At higher contact times the hexafluoropropene that is formed has a chance to react and become lost, while at lower contact times less of the chloroheXa fluoropropane becomes dehydrochlorinated.

In calculating the contact time the volume per minute of gas at room temperature fed to the reactor, the volume fraction of the hexafiuoropropene product in the causticscrubbed reactor efiluent, and the volume of the heated reactor zone are calculated. Analysis in volume percent of the reactor effluent that has been washed to remove the hydrogen chloride gives directly the extent of the conversion of 2-chloro-l,l,l,3,3,3-hexafluoropropane, to hexafluoropropene, since the pyrolysis reaction is essentially represented by the following equation:

CFQCHCHCFQ CF3CF=CFZ HCl When, for example, 10* volumes per minute of gas are fed to the reactor and the washed efiluent has a content of 50% hexafluoropropene, the volume of the reaction mixture at the end of the reaction before the hydrogen chloride is removed is 15 volumes, i.e., 5 volumes unreacted chlorohexafluoropropane and 5 volumes of hexafluoropropene plus 5 volumes of hydrogen chloride. The average of the volume of the gas fed to reactor and the volume received from the reactor is taken for the present purposes of calculation. In the example just cited, the average is that of 10 volumes of gas feed and 15 volumes of the reacted mixture or 12.5 volumes per minute for purposes of calculation. Thus a correction must be made for the increased volume of the reaction mass during the pyrolysis, because the change in the volume reduces the contact time in proportion to the extent of the change. Such a correction amounts to one-half of the volume fraction of the hexafinoropropene content of the washed efiluent gas. To illustrate the calculation of contact specifically for the instant requirements take the first line of data in Table 2 infra column 4. The feed rate is 6 cc. per minute. The correction to he applied is /2 of 0.34 which gives a corrected feed rate of 6 l.l7=7.02 cc. per minute. Since the volume of the 254 mm. heated section of the 6.3 mm. tube is 7.9 cc., the contact time for 3 this feed rate, these tube dimensions, and conversion is 67 seconds, i.e., 60 divided by 7.02/79.

The preferred process of this invention is continuous wherein the material to be pyrolyzed is continuously fed into the reactor to allow a limited time of exposure to the action of the heat while the reaction products are continuously withdrawn from the pyrolysis tube. However, limited batches of hexafluoropropene can be prepared provided the batch is moved through the reactor within the requisite time-period.

The following representative examples are given by way of illustration.

Example I Through a platinum-lined tube constructed of nickel, chromium and iron alloy having an inside diameter of 6.3 mm. and heated for 254 mm. of its length to between 706 C. and 713 C. by an electric furnace was passed a mixture of 2-chloro-1,l,1,3,3,3-hexafluoropropane and nitrogen. These gases were metered to deliver to the reactor 22 cc. per minute of the chlorohexafiuoropropane and 10 cc. per minute of nitrogen measured at room temperature and atmospheric pressure. On the basis of a conversion of 30% hexafluoropropene determined by analysis of the end products the contact time was about 13 seconds. Time of flight mass spectrometry identified the reaction product essentially as hexafluoropropene.

Example 2 2 chloro 1,1,1,3,3,3-hexafluoropropane was passed through the reactor used in Example 1 at a rate of 14 cc. per minute while the temperature was varied from 400 C. to 800 C. The effluent gas was scrubbed with 10% aqueous potassium hydroxide solution, dried by passing through a bed of anhydrous calcium sulfate, and then analyzed by vapor phase chromatography for its hexafiuoropropene content. No reaction took place between 400 C. and 640 C. The contact time as calculated above with a conversion varying from zero to forty percent varied from 34 seconds to 28 seconds. Above 640 C. hexafluoropropene formed to the extents shown in Table 1.

TABLE 1.EFFECT OF TEMPERATURE ON FORMATION OF HEXAFLUOROPROPENE BY PYROLYSIS OF 2- CHLORO-l,1,1,3,3,3-HEXAFLUOROPROPANE Conversion, in volume percent Temperature of (percent hexafluoropropene in reaction, C.: washed reactor elfluent) 680 13 725 24 750 37 733 37 800 18 Under the above operating conditions the optimum temperature range for the formation of hexaflu'oropropene is from about 750 C. to about 775 C.

4 Example 3 By the procedure of Example 1, 2-chloro-1,1,1,3,3,3- hexafi'uoropropane was fed at varying rates into the same pyrolysis tube heated to 735 C. The reactor efiluent was scrubbed, dried, and then analyzed for hexafluoropropene as in Example 2 with the results given in Table 2.

TABLE 2.EFFECT OF FEED RATE ON FORMATION OF Thus the optimum feed rate expressed as a contact time is about 30 seconds as indicated above. At low feed rates and consequently high contact times a portion of the hexafiuoropropene that is formed is pyrolyzed to other fluorine containing compounds.

The hexafluoropropene produced by this process shows a high degree of chemical reactivity and has utility as a chemical intermediate, for example in the preparation of various halogenated propanes. It is useful as a monomer for preparing fluorine containing polymers and c0- polymers.

As many apparently widely dilferent embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claims.

What is claimed is:

1. A process for producing hexafluoropropene which comprises pyrolyzing 2-chloro-1,1,1,3,3,3-hexafluoropropane at a temperature in the range of from about 650 C. to about 850 C., for a time in the range of from about 3 seconds to about 120 seconds and isolating the hexafluoropropene produced.

2. A process for producing hexafluoropropene which comprises pyrolyzing 2-chloro-1,1,1,3,3,3-hexafiuoropropane at a temperature in the range of from about 725 C. to about 775 C., for a time in the range of from about 20 seconds to about 30 seconds and isolating the hexafluoropropene produced.

References Cited UNITED STATES PATENTS DANIEL D. HORWITZ, Primary Examiner. 

